Reduce shared memory usage storing the empty array

[TysonAndre]

Reduce shared memory usage in php 7.3+ for apc.serializer=default. Store the pointer to the immutable zend_empty_array rather than a brand new array. (APCu is already storing a pointer to static variables elsewhere, e.g. uninitialized_bucket)

[nikic]

I don't think this is correct in this form. Not every empty array is a zend_empty_array.

Another thing to keep in mind is that empty arrays have have different next free elements, though I'd have to check whether apcu preserves that, possibly it's already ignored.

[TysonAndre]

I fixed the error - the earlier commit made this use 0 memory, but didn't set the zval to zend_empty_array when persisting.

I don't think this is correct in this form. Not every empty array is a zend_empty_array.

I know, but the question is whether any empty array (in userland arrays) can be replaced with the zend_empty_array without observable effects. On unpersist, this is already using the EMPTY_ARRAY from #323

        case IS_ARRAY:
#if PHP_VERSION_ID >= 70300
            if (Z_ARR_P(zv)->nNumOfElements == 0) {
                ZVAL_EMPTY_ARRAY(zv); /* #323 */
                return;
            }
#endif
            Z_ARR_P(zv) = apc_unpersist_ht(ctxt, Z_ARR_P(zv));
            return;

Another thing to keep in mind is that empty arrays have have different next free elements, though I'd have to check whether apcu preserves that, possibly it's already ignored.

apc_persist_copy_ht ignores that. serialize() functions also ignore that by design.

    if (ht->nNumUsed == 0) {
#if PHP_VERSION_ID >= 70400
        ht->u.flags = HASH_FLAG_UNINITIALIZED;
#else
        ht->u.flags &= ~(HASH_FLAG_INITIALIZED|HASH_FLAG_PACKED);
#endif
        ht->nNextFreeElement = 0;
        ht->nTableMask = HT_MIN_MASK;
        HT_SET_DATA_ADDR(ht, &uninitialized_bucket);
        return ht;
    }